1. Field of the Invention
The present invention relates to an optical transmission system which transmits a plurality of wavelengths of signal light in a wavelength division multiplexing manner.
2. Related Background Art
Silica type optical fibers are used as optical transmission lines in optical transmission systems. Chromatic dispersion becomes zero in the vicinity of a wavelength of 1.3 xcexcm in typical single-mode optical fibers which are most commonly in use among the silica type optical fibers. Hence, signal light in the 1.3-xcexcm wavelength band is used since the waveform of signal light can be restrained from deteriorating due to cumulative chromatic dispersion.
Since the transmission loss of silica type optical fibers becomes the lowest in the vicinity of a wavelength of 1.55 xcexcm, whereas optical fiber amplifiers which can optically amplify signal light in C band (a wavelength band from 1530 nm to 1565 nm) and L band (a wavelength band from 1565 nm to 1625 nm) have come into practical use, the signal light in C and L bands is also in use. In this case, in order to restrain the waveform of signal light from deteriorating due to cumulative chromatic dispersion in these wavelength bands, dispersion-shifted optical fibers whose chromatic dispersion becomes zero in the vicinity of a wavelength of 1.55 xcexcm are favorably used.
Meanwhile, a wavelength division multiplexing (WDM) optical transmission system transmits a plurality of wavelengths of signal light in a wavelength division multiplexing manner, thereby being able to perform communications with a large capacity at a high speed. A larger capacity is demanded therefor, whereas its signal light wavelength band is desired to be expanded. However, the above-mentioned typical single-mode optical fibers may not be suitable for transmitting signal light in the 1.55-xcexcm wavelength band, since their chromatic dispersion has a large absolute value in the 1.55-xcexcm wavelength band. Also, the above-mentioned dispersion-shifted optical fibers may not be suitable for transmitting signal light in the 1.3-xcexcm wavelength band, since their chromatic dispersion has a large absolute value in the 1.3-xcexcm wavelength band.
Therefore, an optical fiber intended for propagating both signal light in the 1.3-xcexcm wavelength band and signal light in the 1.55-xcexcm wavelength band has been proposed (see Japanese Patent Application Laid-Open No. HEI 11-281840). This optical fiber has a zero-dispersion wavelength in the vicinity of the absorption peak caused by OH group, whereas the absolute value of chromatic dispersion is relatively small in each of the 1.3-xcexcm wavelength band and 1.55-xcexcm wavelength band.
Though the optical fiber proposed in the above-mentioned publication is intended for propagating signal light in the 1.3-xcexcm wavelength band in addition to that in the 1.55-xcexcm wavelength band, transmission loss is greater in the 1.3-xcexcm wavelength band than in the 1.55-xcexcm wavelength band. However, no optical amplifier has been known to be favorable and practical as one which can optically amplify signal light in the 1.3-xcexcm wavelength band. Therefore, optical transmission systems using the optical fiber proposed in the above-mentioned publication as an optical transmission line may not be suitable for performing long-distance communications.
In order to overcome the problem mentioned above, it is an object of the present invention to provide an optical transmission system which can perform optical transmissions over a long distance at a low loss by using a plurality of wavelengths of signal light in a wide signal light wavelength band.
The optical transmission system in accordance with the present invention comprises an optical fiber transmission line having a zero-dispersion wavelength of 1350 nm to 1440 nm and a cable cutoff wavelength band of less than 1368 nm, the optical fiber transmission line transmitting signal light at least in a wavelength band from 1450 nm to 1530 nm (S band) and Raman-amplifying the signal light in S band when Raman amplification pumping light is supplied thereto; and Raman amplification pumping light supplying means for supplying the Raman amplification pumping light to the optical fiber transmission line.
According to this optical transmission system, Raman amplification pumping light supplying means supplies Raman amplification pumping light to an optical fiber transmission line. Signal light in S band (a wavelength band from 1450 nm to 1530 nm) is transmitted through the optical fiber transmission line and is Raman-amplified during the transmission. In this optical transmission system, since the cable cutoff wavelength of the optical fiber transmission line is less than 1368 nm, each of the signal light in S band and the Raman amplification pumping light (having a wavelength of 1368 nm to 1439 nm) can propagate through the optical fiber transmission line. Since the zero-dispersion wavelength of the optical fiber transmission line is 1350 nm to 1440 nm, the chromatic dispersion of the optical fiber transmission line in S band becomes at least 0.1 ps/nm/km, so that four-wave mixing is restrained from occurring, whereby this optical transmission system is suitable for propagating a plurality of wavelengths of signal light in S band.
Therefore, in the optical transmission system, a plurality of wavelengths of signal light in S band are Raman-amplified when propagating through the optical fiber transmission line, so that their effective loss is small, whereby repeating sections can be made longer. Since the waveform of signal light is restrained from deteriorating due to four-wave mixing, the power of signal light can be made higher, which also allows repeating sections to become longer.
In the optical transmission system in accordance with the present invention, the optical fiber transmission line may have an effective core area of at least 45 xcexcm2 at a wavelength of 1550 nm. In this case, nonlinear optical phenomena including four-wave mixing are restrained from occurring, so that the deterioration in waveform of signal light is further suppressed, whereby the power of signal light can further be enhanced. Therefore, repeating sections can further be made longer.
In the optical transmission system in accordance with the present invention, the optical fiber transmission line may have a chromatic dispersion slope with an absolute value of 0.065 ps/nm2/km or less at a wavelength of 1550 nm. In this case, the cumulative chromatic dispersion of signal light generated upon the propagation through the optical fiber transmission line can be compensated for by a dispersion compensator provided in a receiving station, for example.
In the optical transmission system in accordance with the present invention, the optical fiber transmission line may have a transmission loss of 0.5 dB/km or less at a wavelength of 1380 nm. In this case, loss is small in the vicinity of a wavelength of 1380 nm at which an absorption peak is caused by OH group, so that the Raman amplification pumping light near this wavelength can propagate through the optical fiber transmission line at a low loss, whereby Raman amplification gain can fully be secured. Therefore, repeating sections can further be made longer.
In the optical transmission system in accordance with the present invention, the optical fiber transmission line may also transmit signal light in a wavelength band from 1530 nm to 1565 nm (C band) or signal light in a wavelength band from 1565 nm to 1625 nm (L band), whereas an Er-doped optical fiber amplifier for optically amplifying signal light in C band or L band when pumping light is supplied thereto may further be provided. In this case, signal light in S band is Raman-amplified in the optical fiber transmission line, while signal light in C or L band is optically amplified by the Er-doped optical fiber amplifier. Therefore, this optical transmission system can perform optical transmissions over a long distance at a low loss by using a plurality of wavelengths of signal light in a wide signal light wavelength band including S, C, and L bands.
In the optical transmission system in accordance with the present invention, the pumping light supplied to the Er-doped optical fiber amplifier may have a wavelength of 980 nm. This case is suitable for transmitting signal light in S band by using pumping light having a wavelength shorter than S band.
In the optical transmission system in accordance with the present invention, the Raman amplification pumping light supplying means may propagate the Raman amplification pumping light from an introducing position for the Raman amplification pumping light into the optical fiber transmission line in a direction opposite from that directed to an Er-doped optical fiber amplifier whose distance from the introducing position along the optical fiber transmission line is the shortest. Since the Raman amplification pumping light can be propagated over a long distance, this case is suitable for Raman-amplifying signal light in S band.